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Letters Long Range Attraction in Water Vapor. Capillary Forces Relevant to “Polywater” V. V. Yaminsky† Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, The Australian National University, Canberra, ACT 0200, Australia Received August 6, 1996. In Final Form: October 1, 1996X A long range attractive force occurs between freshly molten glass surfaces in water vapor. The attraction extends to distances of micrometers. The effect is more pronounced for low melting glasses with a high soda content. It is due to dissolution of the silica matrix components in vapor condensate which forms thick films of aqueous electrolyte on the surfaces. The attraction results from the springing action of the capillary annulus which bridges the surfaces together. The effect occurs in water vapor but not with vapors of other liquids. Some hydrophobic surfaces in water are known to show similar effects.
1. Introduction In the late 1960s considerable excitement was aroused by reports stating that properties of water adsorbed from vapor on freshly molten silica glasses differ vastly from those of bulk water or water films adsorbed on prewashed surfaces. A scandalous hypothesis of a chemical transformation of water into a new compound on virgin highenergy surfaces was later dismissed as an artifact with reference to ionic and silicic acid contamination. The problem closed since then is occasionally recalled as an anecdote.1 But the original experimental observation has never been dismissed. The old polywater experiments were sophisticated. A simple test can be suggested which readily convinces one that the surface of a freshly molten glass is indeed different from that of glass which had ever been rinsed with water. For example, solution of CTAB (a common surfactant used to render silica hydrophobic) spreads onto the surface of flame polished glass with zero contact angle. If the same solution advances for the first time on a prewashed glass slide, the three-phase line shows a characteristic stick-slip motion during which the contact angle periodically rises to high values. The stick-slip effect is known to be due to the adsorption of the cationic surfactant on the silica surface in air in front of the three-phase line of the advancing liquid.2 The entirely different behavior of a freshly molten glass which exhibits ideal wetting under the same conditions can be explained if one admits that ionic components of commercial soda glasses are present on the surface after melting. Before CTAB can be adsorbed, the soda has first to be washed away, and this is what happens during the first immersion. An even more striking difference in behavior of freshly molten and prewashed silica glasses is shown by surface force measurements. Glass is a traditional substrate for surface force experiments.3-6 By melting glass, smooth †
Telephone (06) 249 4693. FAX (06) 249 0732. E-mail vvy110@ rsphysse.anu.edu.au. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; p 282. (2) Yaminsky, V. V.; Yaminskaya, K. B. Langmuir 1995, 11, 936. (3) Tomlinson, G. Philos. Mag. 1928, 6, 695.
S0743-7463(96)00773-1 CCC: $14.00
and clean surfaces can conveniently be obtained, and these can be further chemically modified in a variety of ways. Experiments over decades were carried out with pure silica but commercial soda glasses were also used. Whatever brand of silica glass is taken for such experiments, the results are typically similar for forces studied in aqueous solutions of various substances including electrolytes and surfactants. However for experiments in water vapor, soda present in the glass can have a dramatic effect. This is what we now demonstrate. In connection with our observation we briefly mention the well-known but mysteriously obscure long range forces of a similar character which act between some hydrophobic surfaces in water. 2. Materials and Methods The material used was Pasteur pipets Flint glass. This glass has a low melting point. Some control experiments were done with other high and low soda content glasses including wellknown commercial brands of Pyrex and pure fused quartz. The necks of the pipets and other low melting temperature glasses were melted in propane-air, the Pyrex and silica rods in propane-oxygen flame; they were installed in an interfacial gauge7 while hot. The measurements started several minutes later, after the samples were cooled to room temperature (around 22 °C). A sample typically represents a sphere of a few millimeters in diameter at the end of a smaller diameter cylinder. One of the samples is mounted on a piezoelectric force sensor which is loaded with a magnet and a coil while the other sample is fixed. The mechanical stiffness (k) of the sensor spring is 400 N/m. The amplitude of the external load up to 10 mN corresponds to a displacement amplitude of 25 µ. The load is typically applied in triangle ramps at a given frequency, with a period which was varied from seconds to minutes. The hardware included a Mark IV SFA top plate arrangement upside down covered with a crystallizing dish. This chamber was not hermetically sealed but could be conveniently removed (4) Bradley, R. S. Philos. Mag. 1932, 13, 853. (5) Yaminsky, V. V.; Yusupov, R. K.; Amelina, E. A.; Pchelin, V. A.; Shchukin, E. D. Surface Tension at Solid-Liquid Boundaries. Cohesive Forces between Smooth Elastic Particles. Kolloid. Zh. 1975, 37, 918925 (in Russian; English translation in: Colloid J. USSR 1975, 37, 824-829). (6) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (7) Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. Langmuir 1996, 12, 3531.
© 1997 American Chemical Society
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Figure 1. Interaction between two soda glass spheres in water vapor. An irregularity seen on hitting the hard wall after the jump is a vibration caused by the impact. A small force discontinuity at a distance about 450 nm can be noticed (see discussion in section 3.3). and replaced in a few seconds without interrupting the data collection. The cover dish, which substituted for the native chamber of the apparatus, served to give protection against air convection and dust contamination but also allowed maintaining vapor close to saturation by placing a small beaker with water under it. Alternatively, a drying agent was placed under the cover, in some experiments.
3. Results 3.1. Preliminary Considerations. A surface force experiment typically begins with a control measurement in air. For molten silica as for any other substrate, van der Waals attraction dominates interaction in air. The attraction causes the surfaces to jump into adhesive contact from a distance of the order of 10 nm, the exact value depending on the strength of the attraction and the mechanical stiffness of the dynamometric spring. We have noticed, however, that under several circumstances surfaces experienced attraction from distances orders of magnitude greater than this. This could not be attributed to van der Waals forces (Figure 1). Such an observation, which might be discarded as an “artifact”, we explored in some detail. The suspicion that contact electrification might be involved can be dismissed. Ionization of air to neutralize static charge if present was found to have no effect on the attraction. The only other known force which would cause an attraction of a similar range is the capillary force. While humidity of ambient air is typically less than 100%, capillary condensation becomes a plausible candidate if a layer of salt is present. The hygroscopic ionic material contained in the silica matrix and segregated on the surface readily forms a solution. It covers the hydrophilic substrate with a film in which dry patches may occur or the solution may form microdroplets if the contact angle is larger than zero. The film of such an aqueous condensate can be quite thick compared to ordinary wetting films of, e.g., pure organic liquids adsorbed on mica.8,9 It coexists with humid air at a relative pressure of water vapor smaller than unity, which would not be possible for a monocomponent film. (8) Beaglehole, D.; Christenson, H. K. J. Phys. Chem. 1992, 96, 3395. (9) Christenson, H. K. Phys. Rev. Lett. 1994, 73, 1821.
If a liquid meniscus bridging the two surfaces is formed, it is then stable to very large distances. Indeed, the Kelvin equation which accounts for capillary effects in liquidvapor equilibria diverges at coexistence. The coexistence here occurs when the vapor is undersaturated with respect to pure water. Through this effect the capillary attraction can become extraordinarily long ranged. We report here on several observations related to this effect. 3.2. Experimental Observations. Before we proceed to measurements in vapor we note first that results on the Flint glass in air shortly after melting show variability. Pull-off forces at different contact positions and for different samples range considerably, between 20 and 50 mN/m in most cases, but smaller and large values occur occasionally. Forces are scaled as usual by 2πR, where R is the mean radius of the interacting surfaces. For the weak forces-hard body limit this scaling is “Derjaguin approximation”, which converts forces to energies per unit area.10 The factor changes to 1.5π for pull-off forces between soft-dry macroscopic bodies.11 It is generally not constant when the contact deformation is not negligible on the scale of the effective range of the surface forces. A large variability of pull-off forces much greater than the 25% uncertainty between Derjaguin and JKR factors is typical of a surface force experiment in dry atmosphere using mica surfaces. Here the scatter is attributed to much larger time of the sophisticated sample preparation and data collection.12 The changes to the force occur when the contact position is changed, or from samples to samples. The interaction is reproducible for consecutive measurements at a given spot. In many such measurements an ordinary van der Waals jump into contact is observed. In other cases a steep repulsion occurs between 20 and 10 nm. Adhesive contact between the surfaces occurs after surmounting the barrier. In other cases the barrier is absent but the observed jump into contact is several times larger than a van der Waals jump. After long term residence in humid air a low gradient attraction can be measured in a range of larger distances. Here the force is experimentally significant while dF/dD < k. At a smaller distance when the force gradient becomes greater than the stiffness, the spring becomes unstable and jump into contact occurs. These observations are characteristic of experiments with freshly molten soda glass samples in ambient air. Different interaction patterns might be attributed to a nonuniform surface structure which then would depend upon exact conditions of the flame treatment during melting. The effects of humidity are to smoothen such a scatter by formation of contact condensates. This common explanation does not yet account for the fact that there are no uncertainties if the samples were rinsed with water before the experiment and then dried. Pull-off forces in this latter case are reproducible, and the values are high, in the range 80-100 mN/m. Further, there is no barrier before the jump into contact and the jump-in distance is consistent with the spring stiffness and the van der Waals estimate. The results are the same for different contact positions. We are compelled to assume that the diversity in the forces observed for freshly molten samples is not due to intrinsic roughness of the silica glass substrate. The latter appears be extremely smooth. The scatter is rather due to segregated ionic components at the surface. Some evidence for such a segregation has been reported for freshly cleaved mica surfaces for which microcrystals of potassium carbonate are possibly formed by surface (10) Derjaguin, B. V. Kolloid Z. 1934, 69, 155. (11) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A324, 301. (12) Christenson, H. K. J. Phys. Chem. 1993, 46, 12041.
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hydrolysis and by subsequent reaction of the sodium hydroxide with carbon dioxide in ambient atmosphere.13 No changes to the interaction pattern of prewashed surfaces occur if phosphorus pentaoxide, used here as a drying agent, is placed inside the chamber. If, instead, a beaker with water is placed inside the apparatus and saturates the chamber with vapor, there is a noticeable increase of the jump-in distance, by about 5 nm. This is what is expected in the presence of adsorbed films of pure wetting liquids. When the vapor pressure is close to saturation, these can be a few nanometers thick. The jump-in distance increases, by a value on the order of the film thickness as is predicted theoretically by the effect of van der Waals forces14 and shown by observation.9 Adhesion decreases slightly, to 70-75 mN/m, i.e., to the value of the surface tension of pure water. This is also an expected result. Indeed, capillary attraction is the only force that determines the adhesion in water vapor. This is because adhesion between silica surfaces across liquid water in the meniscus which bridges the two surfaces equals zero. For capillary condensates with radii much smaller than R, which is always the case for macroscopic samples, the maximum value of the capillary force (F), reached when the surfaces are at contact, follows from the well-known expression10 F/(2πR) ) γ, where γ is the surface tension of the liquid. The Derjaguin approximation is justified for capillary forces because contact deformation remains small compared to the cross section area of the meniscus over which the capillary pressure acts. The interaction pattern for prewashed surfaces is thus well explained in terms of water adsorption on hydrophilic surfaces. If, however, water vapor saturates the chamber in the experiment with freshly molten glass samples, the results are dramatically different. With runs into contact continued at a prescribed frequency, the jump-in distance increases with time after the introduction of water vapor. It becomes larger from cycle to cycle. After several minutes, a stationary value is reached. For Flint spheres this is about half a micrometer (Figure 2a). This is almost a hundred times larger than both the van der Waals jump in dry atmosphere and the slightly increased jump-in observed with prewashed surfaces in water vapor. Pull-off forces also increase progressively over this period of time (Figure 2b). The limiting values reached after about 5 min are essentially as high as and as reproducible as those for prewashed surfaces. For the latter the adhesion forces are reduced by a small amount by water adsorption and the jump-in distances are only slightly larger in saturated water vapor than in dry atmosphere with no long range component. For freshly molten surfaces while the jump-in distance increases greatly with time in water vapor the interaction undergoes another remarkable transformation. As long as the increasing jump-in distance remains below some critical value, there is no noticeable interaction before the jump-in occurs. However, after a number of cycles the next cycle suddenly shows up an attraction which gradually increases when the surfaces approach each other. This attraction arises at submicrometer or even micrometer distances, which are several times larger than the critical distance at which the jump-in instability occurs. While the low gradient attraction is enhanced immensely at larger distances, the jump-in distance typically becomes somewhat smaller immediately after this transition. Having once appeared, this super long range attraction (13) Christenson, H. K.; Israelachvili, J. N. J. Colloid Interface Sci. 1987, 117, 576. (14) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1976, 54, 157.
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Figure 2. The jump-in distance (a) and the pull-off force (b) for Flint glass spheres vs time in water vapor. Two subsequent jumps occur in measurements carried out during an interval between 400 and 500 s; the upper points (filled diamonds) correspond to a sudden onset of attraction via coalescence. At larger times the force discontinuity is no more present and only one instability is left which occurs when the gradient of the preceding long-range attraction exceeds the stiffness of the spring.
then shows up again in every subsequent cycle. The strength and the range of the attraction and the value of the jump-in distance increase and stabilize after several minutes. In some experiments, after the surfaces, exposed for a few minutes to water vapor, are in contact we observed a sudden increase of the apparent intersurface separation by about 10 nm. This typically happens on unloading in the cycle which precedes immediately the cycle in which the long range attraction first appears. Occasionally such a step can occur also in one of the later cycles. Then after the surfaces are separated, on subsequent approach the jump-in distance symptomatically shows a larger increase over the ordinary trend. This results in characteristic
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steps in otherwise smooth jump-in distance vs time plots (one such step occurs in Figure 2a between 300 and 400 s). 3.3. Interpretation. Qualitative interpretation of these curious effects observed for freshly molten soda glasses in water vapor seems fairly clear. A layer of water soluble components from the glass may be segregated on the surface during melting. Water can also hydrolyze sodium silicate of the glass matrix. As the bulk glass is essentially insoluble in water, the leaching of ions is restricted to a more or less thin surface layer. The amount of soda released during the first few minutes of exposure to water vapor then remains constant. Water vapor adsorbs on the surface, leaches soda, and dissolves the segregated ionic material. Initially the vapor is strongly supersaturated with respect to the concentrated electrolyte solution which there forms. The film thickness grows rapidly, well in excess of the values for adsorbed films of pure water on clean, e.g., prewashed or pure silica surfaces. Much weaker but otherwise similar effects apparently occur for potassium ions on freshly cleaved mica. The amount of the surface electrolyte is here smaller. Nevertheless, the above mentioned wetting test with CTAB works for freshly cleaved mica as well (also cf., e.g., ref 15). As long as the water vapor remains supersaturated with respect to the concentrated electrolyte, the film continues to grow. However, as the solution within the film dilutes itself by absorbing more and more water vapor, this process also slows down. The film does not grow further when the equilibrium vapor pressure of the solution within the film, which increases when the film thickens and concentration decreases, becomes equal to the acting vapor pressure in the chamber. The exact value for the equilibrium thickness should depend both on the amount of sodium hydroxide and other electrolyte segregated by melting and/or released by leaching and the acting humidity and on the presence of CO2. An independent and exact control for surface composition would be needed to allow quantitative estimates. However, the measured jump-in distances at which coalescence occurs and the bridge forms give an idea of the magnitude of thicknesses involved. Theoretically, for a monolayer of a dry salt of an average thickness of a few angstroms to be converted to a 1% solution, for which the vapor pressure approaches that of pure water, film thicknesses of several hundred angstroms are needed. These estimated thicknesses of the aqueous electrolyte films are proportionally higher if the amount of soluble material segregated by the melting and extracted by water is larger (cf. e.g. ref 16). On a first approach the films coalesce when the distance between the surfaces becomes comparable to the thickness of the films. After the coalescence has taken place, the condensate that forms the films then drains into the contact slit between the spheres under the effect of negative capillary pressure of the concave liquid meniscus. This meniscus readily grows, by pumping the liquid out of the films and absorbing the vapor, to much greater sizes than for ordinary capillary condensates of pure water between hydrophilic surfaces or of wetting organic liquids. When the surfaces are separated, the capillary bridge is stable up to a distance which is generally much larger than the coalescence distance at which it forms on approach. If the film is in equilibrium with the vapor at a given value of the pressure, the bridge vanishes at a distance dictated by the Kelvin equation. (15) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (16) Pashley, R. M. J. Colloid Interface Sci. 1980, 78, 246.
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For pure liquids the Kelvin radius diverges at saturation. However, when a film contains a fixed amount of involatile solute, the saturation pressure for the solution is reduced. Then when the acting vapor pressure is somewhat lower than the equilibrium saturation value for pure bulk solvent, the equilibrium rupture distance can be very large but remains finite. This occurs because the amount of water condensed in the gap increases when the distance between the surfaces increases. Since the amount of salt present remains unchanged, the concentration of the solution decreases. Its saturating pressure increases, the condensation slows down, and the bridge evaporates. The condensation-evaporation equilibrium could be more difficult to maintain during measurements at short periods and for less volatile films of concentrated solutions. Then the volume of the condensate does not change when the distance between the surfaces increases. The capillary joint is ruptured at a smaller distance due to the critical mechanical instability. The long range attraction which precedes the jump into contact arises when the films on the surfaces become sufficiently thick. Thicker films form bigger necks. Through this, the rupture distance increases and at some point exceeds the distance amplitude of the loading ramps. Since then the bridge does not break any more. The bridge now acts as a capillary liquid spring which joins the two surfaces and elongates and contracts reversibly without being disconnected while the samples are moved backward and forward during the measuring cycles. At large distances when the elongated neck thins, the absolute magnitude of the force and the gradient of the attraction are small. This gives the impression that the force vanishes asymptotically at such distances. In our case the free part of the ramp amplitude when the surfaces are out of contact is several micrometers. The attraction noticeable over most of this range is the capillary force. The force is small at the upper side of the range and increases progressively with decreasing distance. The rapid jump into contact occurs when the gradient of the attraction exceeds the constant of the spring. The jump out of contact occurs when the applied unloading force exceeds the maximum value of the capillary attraction which is 2πRγ. The surfaces jump apart to a large distance determined by the ratio of this value to the stiffness of the dynamometric spring. At such distances the liquid bridge is still present but the capillary force reduces to vanishingly small values. If the surfaces are moved intentionally further apart so that the critical rupture distance is exceeded, then the liquid neck breaks. On the next subsequent approach there is now no perceptible interaction before the jumpin. This jump can occur from a distance larger than that for the jump which occurs without breaking the bridge. The films on the two surfaces separated by an air gap first jump toward each other under the effect of van der Waals forces and coalesce. For fully developed polywater layers on Flint glass, this jump occurs over a distance of almost 100 nm. After the bridge is formed, it induces capillary attraction which is the same as in uninterrupted cycles. Over a range of distances the system is in mechanical equilibrium. When the gradient of the capillary force exceeds the rigidity of the spring, there is a second jump which occurs as for continuous cycles. It brings the surfaces into contact. A similar two-step jump-in may occur if the growing film becomes sufficiently thick while the critical rupture distance remains smaller than the ramp amplitude. Depending on the parameters the first jump at larger
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distances can be small. In this case the capillary force is small at the distance where the bridge forms. It then increases and shows itself over a range of smaller distances before a second jump finally brings the surfaces into contact. We observed this type of behavior for some glasses which give thinner films. For smaller annuli formed by such films, the critical rupture distance is also smaller so that the neck breaks during separation in each cycle. When coalescence on approach occurs, the force increases from almost zero to a small finite value. The force discontinuity is small. Such a step at the limit of force resolution can be noticed in Figure 1. Here at a distance about 450 nm the bridge is formed. With a step below the resolution limit the event will not be marked on the force curve. In the limit of large distances the force-distance profile then gives an impression of asymptotic behavior. In the example in Figure 1 the attraction becomes noticeable below 450 nm. It then increases progressively with decreasing distance. It can be measured down to 100 nm. From here mechanical stability is lost and the second jump into contact occurs. Exact distance law of the capillary attraction depends on exact conditions imposed on the condensate. These can vary between the constant chemical potential and constant volume limits. We do not go here into theoretical details. Capillary annuli of pure liquids have been analyzed by means of the Laplace equation in several works, and a similar approach applies for bridging vapor cavities in nonwetting liquids (see, e.g., ref 17 and references therein). With these matters established we note here that the distance dependence observed for the capillary attraction caused by “polywater” in water vapor is indeed similar to that also observed experimentally for a vapor cavity between two hydrophobic surfaces of silylated glass under liquid water.18 Interestingly, force-distance plots for this attraction are very similar to the long range attraction observed in water between mica surfaces covered with deposited Langmuir-Blodgett monolayers.19,20 In several reported cases21 the hydrophobic attraction is almost as long range as the capillary attraction observed with Flint glass in water vapor. We have already considered fundamental likeness between hydrophobic attraction and capillary condensation phenomena.22,23 Vapor cavities are not necessarily present.19,20 The bridging material is the lipid. The effect caused by the loosely anchored and laterally mobile hydrophobic layers24,25 that slide along the hydrophilic substrate effectively lubricated by water into contact where they form interferometrically “transparent” bridging annuli is considered in great detail separately.26 3.4. Further Observations on Polywater. With these remarks we now proceed to some further experimental observations on capillary forces between glasses (17) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96, 307. (18) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (19) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (20) Christenson, H. K. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992; p 29. (21) Kurihara, K.; Kato, S.; Kunitake, T. Chem. Lett. 1990, 1555. (22) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashley, R. M. Langmuir 1996, 12, 1936. (23) Yaminsky, V. V.; Ninham, B. W. Langmuir, in press. (24) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid Interface Sci. 1993, 161, 91. (25) Yaminsky, V. V.; Nylander, T.; Ninham, B. W. Submitted for publication in Langmuir. (26) Yaminsky, V. V. Colloids Surf., in press.
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in water vapor. As long as the vapor is present and maintained near saturation, the micrometer range attractive effect persists indefinitely. However, if the beaker with water is removed and the drier ambient air is admitted into the chamber, the range of the attraction decreases and the jump-in distance decreases. This is what is expected when the aqueous films become thinner. A kinetic examination shows how the effect of instantaneously decreased humidity which then depends on the conditions of drying evolves. The jump-in distance decreases gradually in time. A manyfold decrease takes place in several minutes. As the solution becomes more concentrated, it dries more and more slowly. If the ambient humidity is low enough the solubility limit eventually will be reached and salt will crystallize out of water again. The precise effect can depend on ambient humidity, convection, the conditions of melting, and the chemical composition of the glass with different ionic species and liberated silicic acid all involved. These factors determine the extent of reversibility and the final state achieved after freshly molten glass has been exposed to water vapor and then dried. The final state after drying can be different from the initial state after melting. For example, ions pumped out of the silica matrix during leaching may be unable to exchange back again. This is for kinetic reasons and because the silicic acid released during leaching could then have undergone condensation into silica. It is this surface silica layer on the top of the bulk substrate which apparently prevents glass from total dissolution and makes surface properties of various glasses similar after washing with water. The drying process is much faster if immediately after water has been taken away phosphorus pentaoxide is placed under the cover. Interestingly, adhesion forces increase over the first minute of this fast drying. Only several minutes later do they become smaller again. The capillary contracting force is proportional to the surface tension which increases when concentration of electrolyte in water increases. This is what first occurs when water from the solution within the film is allowed to evaporate. As the condensate becomes viscous at a later stage and solidifies, the contact strength becomes further larger. It is now not simply due to the surface tension effects of the capillary linkage but rather due to nonequilibrium effects of kinetic resistance and static strength of the hardening material of this bridge which can include salt and silicic acid released during leaching. As the film dries the surface apparently becomes nonuniform. Because of this adhesion again decreases at longer times in dry air and eventually falls to zero. Even if the surface layers still contain substantial amounts of water as occurs in equilibrium with air at normal ambient humidity, the film attains a gel-like state and the interaction is no longer mechanically reversible. The adhesion though reduced and scattered does not disappear entirely. The approach shows irregular and irreproducible long range patterns which contain elements of attraction and repulsion before the hard-wall contact. The solidification effect is particularly dramatic if the ramp period is increased from 10-20 s to several minutes. It is now sufficiently long so that in the presence of phosphorus pentaoxide, the film could dry out while the surfaces remain in contact. What then happens in some instances is that the surfaces are suddenly stuck together very strongly and no longer jump apart. The condensate drying in contact acts here as a silicate glue. It is a wellknown fact that when a silica sol gelates and then dries it converts itself into a porous stone of tremendous strength. In parallel to this observation we observed in our macroscopic glass balls experiments contacts for which
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While interaction after drying occurs under essentially nonequilibrium conditions, the effect of high humidity is reversible. If the surfaces which were dried are placed in saturated water vapor again, the long range interaction pattern is restored. These cycles of drying and swelling can be repeated many times, and each time the attraction disappears on drying and reappears in water vapor again. However, once the surfaces were dipped in liquid water, they lose all such curious properties and show an ordinary van der Waals behavior. The long range attraction no longer arises in vapor any more. In the above examples we often referred to the “Flint glass”, but there is nothing really particular about this. Different soda glasses (and the one in Figure 1 is not “Flint” but some other commercial pipet) show similar effects. However, the higher the melting point of the glass, the smaller is the range of the attraction. Reduction of the melting point of glasses is achieved by adding soda to quartz sand during manufacture. Pyrex has the lowest soda content among other glasses and shows the least pronounced enhancement of the capillary attraction. The effect is almost absent for fused quartz. This chemically pure silica after melting shows much shorter range van der Waals interaction patterns typical of a prewashed soda glass. This is consistent with the observation of Bradley4 who showed that finite adhesion is retained on evacuation for fused quartz but for vitreous spheres prepared by melting of sodium pyroborate pull-off forces in vacuum become small, scattered, and irreproducible. Sodium pyroborate is a hygroscopic compound which forms glasses on its own and is a common component of silica glasses. 4. Conclusions
Figure 3. jump-in distance (a) and the pull-off force (b) for Flint glass spheres equilibrated in water vapor vs time after admitting vapor of ethanol. The long range attraction preceding the jump-in disappears after about 1 min of the exposure.
pull-off forces were as large as 500 mN/m in the scaled representation. These are at least an order of magnitude larger than the attraction observed in reversible systems. After such a contact is destroyed, subsequent adhesion is typically small, indicative of an irregular fracture surface which occurs where the solidified joint gets broken. All these surprising effects are caused by water vapor but not by vapors of ethanol or other liquids. In fact ethanol for example acts rather as a drying agent. This can be demonstrated by adding a beaker with ethanol under the cover alongside of the beaker of water. The jump-in distance decreases and the attraction diminishes (Figure 3). The polywater force disappears in less than 10 min in a fashion similar to that in which it appeared before, when water was introduced. Adhesion becomes smaller in parallel with the smaller surface tension of ethanol which adsorbs on the surfaces and dilutes and replaces water. At larger times the jump-in distance becomes almost as small as that in dry air even though the beaker with water, which saturates the volume with water vapor, is still present under the cover!
The heated discussion which followed the polywater saga ended with the dismissal and assignment of the effect to contaminants. It is not without interest that Isaac Newton himself tried to measure capillary forces. In Art. 31 of the Principia he reported failure, because as he said surface contaminants were owing.27 “Contaminants” which either “cancel” forces or enhance them tremendously are real and ubiquitous. “Artifacts” through which capillary attraction varies enormously are real physical effects of great principal and practical significance. Intriguing observations include segregation and leaching of ions in multicomponent glasses. Vapor condensation accompanied by dissolution is involved in numerous transformations of hygroscopic materials from clays to proteins and nucleation of atmospheric condensates. The basic principles demonstrated here are not restricted to soluble inorganic components of silica glasses in water vapor. A similar attraction due to monolayers of lipids on mica under water is related to cell adhesion and other biological problems. A number of experiments that have been ascribed to new non-DLVO forces to include the long range hydrophobic attraction have in their underlying mechanism essentially the same contact bridging effects that occur for hygroscopic condensates misinterpreted as polywater. Delusions belong to the history and sociology of science. The great mistake of B. V. Derjaguin, interpreted properly, it seems, does have positive future development. LA960773U (27) The reference to Newton’s work has been pointed out by Barry Ninham. Important contributions from him to the paper are numerous and his discussions and advise are appreciated.
